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In Coleman's paper, "Quantum sine-Gordon equation as the massive Thirring Model" (Link to the PRD paper, he pointed out that the massless Thirring Model is sensible only for coupling $g>-\pi$. I do not understand why this is true. Can any one offer me an explanation to this? Is there relevant paper discussing this issue?

Note the Lagrangian is $\cal L = \bar{\psi}i\partial_{\mu}\gamma^{\mu}\psi-\frac{g}{2}j_{\mu}j^{\mu}$, here $j_{\mu}$ is the usual current.

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